This disclosure relates generally to patient monitoring. More particularly, the present invention relates to monitoring of respiratory gas levels of mechanically ventilated patients/subjects. The respiratory gas monitored is typically carbon dioxide.
Carbon dioxide (CO2), which is a byproduct of cell metabolism, is diffused out of the cells to the vascular system and carried by venous circulation to the lungs where it is diffused across the alveolar capillary membrane and exhaled out of the body. Capnometry refers to the (non-invasive) measurement and display of concentration of carbon dioxide in respiratory gases, while a capnometer refers to a machine that produces the CO2 waveforms of respiratory gases. Capnometers measure the concentration of CO2 exhaled at the end of the breath, commonly known as end-tidal breath CO2 (ETCO2). ETCO2 is expressed as a percentage or partial pressure of CO2 in the respiratory gases. Normal values are between 5% and 6%, which is equivalent to 35-45 mmHg. FIG. 1 illustrates a regular time capnogram, i.e. ETCO2 waveform, of a normally breathing subject. A time capnogram comprises two basic segments, an inspiratory segment and an expiratory segment. During the first portion of expiration (time period 1), CO2 level remains zero as the initial gas sampled by the sensor will be from a so-called dead space. As the expiration continues, CO2 level rises to the above-mentioned normal level as the CO2 rich gases from the alveoli reach the sensor (time period 2). At the end of the expiration (time period 3), CO2 level drops to zero as the subject starts to inspire CO2 free gases.
At present, a capnometer is a standard tool for monitoring CO2 levels of subjects in anesthesia and intensive care, for example. This is because CO2 levels and waveforms provide rapid and reliable information that helps to detect and prevent various life threatening events, such as malposition of tracheal tubes and failures in metabolic, cardiovascular and respiratory systems.
Total ventilation may be divided between two parts: the respiratory gases that exchange with pulmonary blood and the respiratory gases that do not exchange with the pulmonary blood. The former is commonly called pulmonary ventilation, while the latter is commonly called dead space ventilation. Dead space thus refers to the respiratory gases that are inhaled but which do not take part in the gas exchange. Physiological dead space may be divided into anatomical dead space and alveolar dead space. Anatomical dead space comprises the gases in the upper airways, such as mouth and trachea, which do not come into contact with the alveoli of the lungs. Alveolar dead space comprises the gases that come into contact with the alveoli without any gas exchange, i.e. without any perfusion taking place. A third form of dead space is commonly termed mechanical or equipment dead space. This is formed by the gases that fill the breathing circuits of a mechanical ventilator system without participating in the gas exchange.
Dead space tends to decrease the ETCO2 readings since the “dead” gas/air that does not participate in gas exchange mixes with the expired gases and thus dilutes the expired CO2. In other words, all the dead space gas in anatomical and equipment dead spaces is not normally exhaled in the beginning of the expiration period but part of the dead space gas mixes with the exhaled CO2 rich gases and dilutes the expired CO2. This may in turn deteriorate the reliability of the correlation with the blood gas CO2 concentrations and lead to underestimation of the arterial CO2 level. Generally, the smaller the patient the greater the effect of dead space. In small patients, the accuracy of capnometry has been increased by using small-volume endotracheal tube connectors and/or using special endotracheal tubes that allow CO2 samples to be taken from the tip of the tube (instead of a regular mouth sensor).
A further factor that may affect the accuracy of the ETCO2 measurement and thus also the reliability of the correlation between ETCO2 and blood CO2 is the respiration rate. As the respiration rate increases, the inspiration and expiration periods shorten and the expiration period may become too short for transferring all CO2 rich gases to the sensor before the next inspiration period starts. This in turn leads to rebreathing, i.e. exhaled gas mixes with the gas in the ventilation system and some of the mixed gas is reinhaled.
Consequently, the accuracy and reliability of the ETCO2 measurement and blood CO2 estimation may become compromised in certain ventilation conditions. As discussed above, the risk of inaccurate ETCO2 measurement is greater when infants and high frequency ventilation are involved. However, high frequency ventilation, which is typically employed to reduce lung injuries or to prevent further lung injuries, may be applied to patients of all ages, from neonates to adults.